A fuel cell is provided in a stack structure in which fuel cell-constituting unit cells each serving as a power generation unit are stacked in a plurality of layers. Each of the unit cells has a membrane electrode assembly sandwiched by opposing separators. In recent years, there has been proposed, e.g. in International Publication No. WO2012/160607, a technique for forming, in a separator central region opposed to a power generation region of the membrane electrode assembly, a fuel-gas gas flow path and a cooling water flow path on top-and-bottom surfaces of the separator by a plurality of pit-and-bump stripes made by press molding or by a plurality of protruded portions.
The cooling water flow path proposed in the above-cited patent reference has cooling water to pass through with changing flow direction by height differences between the protruded portions and the recessed-groove bottom portions. Thus, diffusivity and distributivity of cooling water are improved. On the other hand, when the cooling water passes through parts having height differences such as the protruded portions or the recessed-groove bottom portions with changing the flow direction, the flow of the cooling water may be stagnated. After starting an operation of the fuel cell, no particular obstacle is raised even if the flow of the cooling water has been stagnated since the cooling water has already been delivered over the cooling water flow path so that the flow path is filled with the cooling water. However, after just assemble of the fuel cell is completed, it has been pointed out that the following new problems could occur since air remains in the cooling water flow path.
When the cooling water is supplied after completion of the assemble of the fuel cell, the cooling water passes through with changing the flow direction in an air-mixed state. Therefore, depending on how the flow of the cooling water is stagnated, air can remain in the flow path without being pushed away by the cooling water, and then such air may rise to a vertical upper end of the separator central region. This may, cause an air accumulation. Although such air accumulation can be pushed away in some cases by the supply of the cooling water after starting of an operation of the fuel cell, if the air accumulation still remains at the upper end of the separator central region, hinders the cooling at the air accumulation. Since the above-cited patent reference has no consideration of possibility of an air accumulation, there is need avoiding the air accumulation on the upper end side of the separator central region. In addition, there is need reducing manufacturing costs for the separator having recessed grooves as the cooling water flow path or for the fuel cell.
In order to achieve at least part of the above-described problems, the present invention may be implemented in the following aspects.